Question

I already have a java program using log4j to write output on console, while now I want use another java program to invoke the first one(sub-process) and intercept its output.
I use the method in this page http://www.javaworld.com/javaworld/jw-12-2000/jw-1229-traps.html?page=4, some codes like below

class StreamRedirector extends Thread
{
InputStream is;

StreamRedirector(InputStream is)
{
    this.is = is;
}

public void run()
{
    try
    {
        BufferedReader br = new BufferedReader(new InputStreamReader(is));
        String line=null;
        while ( (line = br.readLine()) != null)
        {
            System.out.println(line);
        }
    }
    catch (IOException e)
    {
        e.printStackTrace();
    }
}
}
.............
try
    {
        Process p = Runtime.getRuntime().exec("run.bat");

        StreamRedirector errSR = new StreamRedirector(p.getErrorStream());

        StreamRedirector outSR = new StreamRedirector(p.getInputStream());

        errSR.run();
        outSR.run();

        int exitVal = p.waitFor();
        System.out.println("Exit Value: " + exitVal);
    }

log4j config:
<appender name="CONSOLE" class="org.apache.log4j.ConsoleAppender">

    <param name="Target" value="System.out" />

    <layout class="org.apache.log4j.PatternLayout">

        <param name="ConversionPattern" value="%d{ABSOLUTE} %-5p  [%t] [%c{1}] %m%n" />
    </layout>
</appender>

I can get the output message, but just in “run.bat” using echo XXX , not the message generated by log4j. If I run “run.bat”, I can see both the output in console. Is there a way that I can also get the log4j output in parent process? thanks.

Answer 1

The StreamRedirector extends Thread but you call run not start. This means that run is getting called synchronously, in the calling thread. Which means that in the following

    errSR.run();
    outSR.run();

The second line does not execute until the first completes. I guess if the error stream does not produce much output then it will eventually complete. But is that really what you intend to do? Instead try

    errSR.start();
    outSR.start();

If the program is using log4j I urge you to consider SocketAppender to grab what is written to it from a different process.

EIDT: Another approach is for StreamRedirector to extend Runnable instead of Thread. The calling code then changes to

Thread errorThread = new Thread(new StreamRedirector(p.getErrorStream()));
....
errorThread.start();

This won’t prevent you from calling errorThread.run() but if you do call it then it will return immediately (instead of blocking as it currently does) suggesting that something is wrong.